Conventionally, as disclosed in the Japanese Patent Laid-Open No. 56-148804 for example, chip resistors of this kind have the cover coat protruding high at a center region on an upper surface of the insulating substrate. When the chip resistor is sucked by a vacuum collet, it is sometimes impossible to suck, or the cover coat is cracked for example, from time to time.
This problem has been solved in a recent chip resistor which is made according to a prior art disclosed in the Japanese Patent Laid-Open No. 8-236302 and as shown in FIG. 1.
Specifically, this chip resistor includes an insulating chip substrate 1 having an upper surface formed with a resistive film 2, a pair of left and right upper electrodes 3 at two ends of the resistive film, a cover coat 4 made of glass for example, covering the resistive film 2, auxiliary upper electrodes 5 on the upper electrodes 3, overlapping the cover coat 4, and side electrodes 6 on a left and a right side surfaces of the insulating substrate 1, making electrical connection with the upper electrodes 3 and the auxiliary upper electrodes 5. With this construction, the cover coat 4 is prevented from protruding or becoming high by the auxiliary upper electrodes 5 formed on the upper electrodes 3.
The insulating substrate 1 has a lower surface formed with a pair of lower electrodes 7 which are electrically connected with the side electrodes 6. The entire surfaces of the auxiliary upper electrodes 5, side electrodes 6 and lower electrodes 7 are coated with metal plate layers 8 made of a nickel plate layer and a solder or tin plate layer formed on the nickel plate layer.
However, according to the prior art, the auxiliary upper electrodes 5 are made just the same way as the upper electrodes 3 are formed at the ends of the resistive film 2, i.e. by first applying an electrically conductive paste of silver (hereinafter simply called silver paste) which is a paste containing silver as a primary component having a low electrical resistance, and then sintering the paste. Although the auxiliary upper electrodes 5 are coated with the metal plate layers 8, the metal plate layers 8 do not have perfect fit to the cover coat, allowing sulfur gases such as hydrogen sulfide in the atmosphere to find ways between the metal plate layer and the cover coat to a region where the auxiliary upper electrodes 5 made from the silver paste overlap the cover coat, causing migration of the metal or other forms of corrosion. When the corrosion reaches the upper electrodes 3, electrical resistance of the resistor is altered, and furthermore, the upper electrodes 3 are electrically disconnected eventually.
The present invention aims at providing a chip resistor which does not have the above problem, and a method of making the chip resistor.